1. Field of the Invention
The present invention relates to a thermal fixing device of an image forming device.
2. Description of the Related Art
Image forming devices, such as laser printers, normally have a thermal fixing device for fixing toner that has been transferred onto a sheet. The thermal fixing device includes a thermal roller and a pressing roller. The thermal fixing device thermally fixes the toner onto the sheet while the sheet passes between the thermal roller and the pressing roller.
The thermal roller of the thermal fixing device normally is a tube. A halogen heater is mounted in the tube following the axial direction of the tube. The halogen heater heats up the tube.
Recently, a thermal fixing device has been proposed wherein the tube is heated directly by induction. A plurality of induction coils are disposed on the tube following the axial direction of the tube. An alternating current if passed through each of the induction coils to generate an induced magnetic flux. The induced magnetic flux induces a current at surface portions of the tube that confront one of the induction coils. Joule heat associated with the induced current heats up the surface portions of the tube. As a result, the entire surface of the tube is heated up directly.
However, the Joule heat that heats the tube can heat up and damage the induction coils on the tube. Also, it is expensive to provide the plurality of induction coils following the axial direction of the tube.
One conventional thermal fixing device includes a single induction coil disposed in confrontation with the entire axial length of the tube. Providing a single induction coil instead of a plurality of induction coils reduces costs. However, in this case the magnetic flux is concentrated at the axial ends of the tube and weaker at the central portion of the tube. As a result, the ends of the tube heat up excessively and the center heats up insufficiently. Such a configuration cannot heat up the tube uniformly.
It is an objective of the present invention to overcome the above-described problems and provide a thermal fixing device that uses induction heating to uniformly heat a thermal region of a thermal member for thermally fixing an image onto a recording medium, the thermal fixing device having a compact and simple configuration, enhanced durability, and reduced cost.
In order to achieve the above-described objectives, a thermal fixing device according to the present invention includes a magnetic circuit configured from an induction coil, a propagation member, and a thermal member. The propagation member is made from a magnetic material that propagates magnetic flux induced by the induction coil. The thermal member has a thermal region made from a magnetic material. The propagation member is magnetically connected to both ends of the thermal region of the thermal member so that the magnetic flux induced by the induction coil heats the thermal region of the thermal member.
With this configuration, when an alternating current is applied to the induction coil, then an induced magnetic flux propagates through the propagation member and an induced current is generated from one end to the other of the thermal region of the heated member. Joule heat associated with the induced current directly heats up the thermal region. For this reason, the entire thermal region can be directly and uniformly heated up across its entire axial length without providing a plurality of induction coils in confrontation with the thermal region across the length of the heated member. Accordingly, the induction coil will not be damaged so that durability can be enhanced. Also, the thermal region of the heated member for thermally fixing the recording medium can be uniformly heated using an induction heating method using only a simple configuration.
It is desirable that the entire thermal region of the thermal member propagate the induced magnetic flux so that magnetic flux is induced across the entire thermal region of the thermal member from one end to the other of the thermal region. As a result, the entire thermal region heats up at the same time. For this reason, the thermal region can be even more uniformly heated up using a simple induction heating configuration.
When the induction coil and the propagation member are stationary and the thermal member is movable, there will be situations when a gap will exist between the thermal member and at least one of the induction coil and the propagation member so that the thermal member can move with respect to the induction coil and the propagation member. The gap will increase the magnetic reluctance between the movable thermal member and the induction coil and the propagation member. In this case, it is desirable to provide a magnetic reluctance reducer for reducing magnetic reluctance between the propagation member and the thermal member so that the induced magnetic flux from the stationary propagation member can be propagated to the thermal region of the movable thermal member. As a result, the magnetic reluctance at the gap can be reduced so that induction heating can be efficiently achieved.
It is desirable that the magnetic reluctance reducer increase the surface area that propagates the induced magnetic flux from the propagation member so that the induced magnetic flux is propagated to the thermal region. As a result, the magnetic reluctance can be reliably reduced and efficient induction heating can be easily and reliably achieved.
It is desirable that the magnetic reluctance reducer serves as a support for the movable thermal member, so that the number of components can be reduced so that induction heating can be performed reliably with a simple configuration.
It is desirable that the induction coil be provided to the outside of the movable thermal member, with respect to the lengthwise direction of the movable thermal member, so that the thermal fixing device can be formed in a more compact shape. Also, with this configuration, the induction coil is less likely to be damaged from heat generated from the thermal region of the thermal member.
When the induction coil is provided around the propagation member at a position external from the movable thermal member in the lengthwise direction of the movable thermal member, it is desirable that the induction coil be installed with respect to the movable thermal member in an integral manner with the connection end portion of the propagation member. With this configuration, during assembly of the thermal fixing device, the induction coil can be provided external from the movable thermal member in the lengthwise direction of the movable thermal member by merely mounting the connection end portion of the propagation member to the movable thermal member. For this reason, the induction coil can be reliably provided to the outside of the movable thermal member in the lengthwise direction of the movable thermal member by a simple assembly process and the thermal fixing device can be made in a more compact shape.
It is desirable that the induction coil be provided around the propagation member at a position external from an axial direction end of the roller so that the thermal fixing device can be made more compact. In this case, it is further desirable that the roller surface have a larger magnetic reluctance than magnetic reluctance of the propagation member so that efficient induction heating can be reliably achieved.
It is desirable that the propagation member be adapted for changing length of a pathway through the thermal region where the propagation member propagates the induced magnetic flux. With this configuration, if the size of the recording medium is changed, then the thermal region can be changed to a size that matches the size of the recording medium by changing the length of the pathway through the thermal region where the propagation member propagates the induced magnetic flux. For this reason, thermal fixing can be appropriately and efficiently performed in accordance with size of the recording medium.
The length of the pathway through which the induced magnetic flux propagates through the thermal region can be changed by configuring the propagation member with first and second propagation members. The first propagation member is magnetically connected to both lengthwise ends of the movable thermal member. The second propagation member is interposed between a non-end portion of the first propagation member and a lengthwise non-end portion of the movable thermal member. The second propagation member is switchably movable between a connection orientation and an interruption orientation. In the connection orientation, the second propagation member magnetically connects the non-end portion of the first propagation member and the lengthwise non-end portion of the movable thermal member. In the interruption orientation, magnetic connection between the non-end portion of the first propagation member and the lengthwise non-end portion of the movable thermal member is interrupted.
With this configuration, when the second propagation member is in the interruption orientation, then the induced magnetic flux propagates through the first propagation member, which is connected to the both lengthwise end portions of the movable thermal member. Therefore, the entire length of the movable thermal member serves as the thermal region. When the second propagation member is in the connection orientation, then the non-end portion of the first propagation member and the lengthwise non-end portion of the movable thermal member are connected so that the portion of the movable thermal member that corresponds to the non-end portion of the first propagation member and the movable thermal member serves as the thermal region. The thermal region can be easily and reliably changed by merely switching the second propagation between its interruption orientation and its connection orientation.
Alternatively, the length of the pathway through which the induced magnetic flux propagates through the thermal region can be changed by configuring a first propagation member to magnetically connect with a lengthwise end of the movable thermal member and a second propagation member to magnetically connect with the other lengthwise end of the movable thermal member. In this case, the first and second propagation members are disposed mutually slidable along lengthwise portions thereof while maintaining magnetic connection therebetween. At least one of the first and second propagation members is slidable in the lengthwise direction of the movable thermal roller while maintained in a magnetically connected condition with the movable thermal roller.
With this configuration, the thermal region can be appropriately changed by merely sliding the at least one of the first and second propagation members by an appropriate amount with respect to the movable thermal member. For this reason, the thermal region can be easily and reliably changed in a continuous manner. Thermal fixation can be performed even more efficiently and appropriately in accordance with the size of the recording medium.
It is desirable that the thermal member be configured from at least one layer of a magnetic material and at least one layer of a material with a thermal conductivity that is higher than thermal conductivity of the layer of magnetic material. Because at least one layer is formed from a magnetic material, the magnetic layer can be properly heated so that proper thermal fixation can be achieved. Also, even if local areas of the magnetic layer are cooled off by the recording medium contacting the thermal member, the heat from other areas will be properly dispersed to the contacted areas because at least one layer is formed from a material with a thermal conductivity that is higher than thermal conductivity of the layer of magnetic material. Therefore, drops in temperature of the thermal member can be prevented. For this reason, thermal fixation can be performed even more efficiently.
In this case, it is desirable that the thermal member includes two outer layers formed from a magnetic material, and an intermediate layer interposed between the outer layers. The intermediate layer is formed from a material with higher thermal conductivity than thermal conductivity of the outer layers. With this configuration, the current induced by the magnetic field occurring by propagation of the induced magnetic flux is generated in the upper and lower layers with the intermediate layer interposed therebetween. Therefore, efficient induction heating can be achieved.
When a casing is provided that covers the movable thermal member, it is desirable that the propagation member be provided integrally with the casing. As a result, configuration can be simplified and costs can be reduced because the number of components is reduced. In this case, it is desirable that the propagation member include a casing-side propagation member and a connection-side propagation member that are separable connected to each other. With this configuration, the casing-side propagation member and the connection-side propagation member of the propagation member separate from each other when the casing is detached from the movable thermal member, so that the casing-side propagation member is detached along with the casing. Therefore, during maintenance for example, there is no need to detach the propagation member in an action separate from the action of detaching the casing. Maintenance can be simplified.
According to another aspect of the present invention, a thermal fixing device includes a thermal member and a magnetic circuit. The thermal member has a thermal region including an outer surface and an inner surface. At least one of the outer surface and the inner surface of the thermal region is formed from a magnetic material. The magnetic circuit generates an eddy current at the at least one of the outer surface and at the inner surface of the thermal region of the thermal member.
By generating an eddy current at at least one of the outer surface and the inner surface of the thermal member, the thermal region, which is formed at least partially from magnetic material in this way, generates heat so that the thermal region can be directly heated. For this reason, the thermal region of the thermal member for performing thermal fixation on a recording medium can be uniformly heated using an induction heating method using only a simple configuration.
According to still another aspect of the present invention, thermal fixing device includes an induction coil, a propagation member, and a thermal roller. The propagation member is made from a magnetic material that propagates magnetic flux induced by the induction coil The thermal roller has a thermal region including an outer peripheral surface and an inner peripheral surface. One or both of the outer peripheral surface and the inner peripheral surface of the thermal region is formed from a magnetic material. The propagation member is magnetically connected to both axial lengthwise ends of magnetic-material ones of the outer peripheral surface and at the inner peripheral surface.
The thermal fixing device according to these aspects of the present invention can be effectively provided to an image forming device including a developing unit for forming the image and a transfer unit for transferring the image onto the recording medium.